1. Technical Field
The present invention relates to a method for producing an L-amino acid using a bacterium. L-amino acids are industrially useful as animal feed additives, health food ingredients, amino acid infusions, and so forth.
2. Background Art
Methods for producing a target substance such as an L-amino acid by fermentation include using a microorganism such as a wild-type microorganism (a wild-type strain), an auxotrophic strain derived from a wild-type strain, a metabolic regulation mutant strain derived from a wild-type strain that may be resistant to various drugs, a strain having properties of both an auxotrophic strain and metabolic regulation mutant strain, and so forth.
In recent years, recombinant DNA techniques have been used in the production of target substances by fermentation. For example, L-amino acid productivity by a microorganism can be improved by increasing expression of a gene encoding an L-amino acid biosynthetic enzyme (U.S. Pat. Nos. 5,168,056 and 5,779,736), or by increasing uptake of a carbon source into the L-amino acid biosynthesis system (U.S. Pat. No. 5,906,925).
Use of carbonate and bicarbonate ions as counter anions to basic amino acids have been disclosed in a method of production of basic amino acids instead of sulfate or chloride ions. The disclosed methods include adding carbonate ions and bicarbonate ions to the medium, controlling the internal pressure of the fermentation tank so that it is positive during the fermentation, or supplying carbon dioxide or a mixed gas containing carbon dioxide to the medium (U.S. Patent Published Application No. 2002/0025564, WO2006/038695).
Conventional amino acid fermentation for members of the L-aspartic acid family, such as L-lysine, is accompanied by the by-production of L-glutamic acid, and in particular, is made even worse by a high pH of the fermentation medium. Since it is often necessary to purify L-amino acids to a high purity level after fermentative production, the presence of by-products is often a problem, and can complicate the purification process ultimately resulting in a reduction of the purity of the desired product.
To date, proteins involved in the uptake of glutamic acid which have been reported in enterobacteria such as Escherichia coli include GltP, GltS (J. Bacteriol., 1992 April; 174 (7):2391-3, J. Biol. Chem., 1990 December; 15; 265 (35):21704-8), GadC (J. Bacteriol., 2006 December; 188 (23):8118-27), and GltIJKL.
It is known that production of L-lysine, L-threonine and L-tryptophan by a strain that has been modified so that the glutamate decarboxylase activity is enhanced can be improved by enhancing the glutamate/GABA anti-porter activity (WO2008/044453). However, there have been no reports of production of L-amino acid using a microorganism in which the gltP and/or gltS genes is/are amplified.